1. Field of the Invention
The present invention relates to a grain-oriented silicon steel sheet suitable for use as the iron core of transformers and other electric machines, and also to a process for producing the same. The silicon steel sheet possesses both good coating properties and good magnetic properties.
2. Description of the Related Art
Grain-oriented silicon steel sheets are used mainly as a material of the iron core of transformers and rotating machines. They are required to have such magnetic properties as high magnetic flux density, low iron loss, and small magnetostriction. Nowadays, there is an increasing demand for grain-oriented silicon steel sheets superior in magnetic properties from the standpoint of energy saving and material saving.
In the production of grain-oriented silicon steel sheets superior in magnetic properties, it is important that the resulting product has a structure such that the grains of secondary recrystallization are densely arranged along the (110)[001] orientation or so-called Goss orientation.
Grain-oriented steel sheets as mentioned above are produced by the following steps. First, grain-oriented silicon steel slabs are produced which contain MnS, MnSe, AlN, BN, or the like as an inhibitor necessary for secondary recrystallization. After heating, they undergo hot rolling. The resulting hot-rolled sheets undergo annealing, if necessary, and then undergo cold rolling (down to the final thickness) once or twice or more, with any intermediate annealing interposed. The cold-rolled sheets undergo decarburization annealing. With an annealing separator (composed mainly of MgO) coated, the steel sheets undergo final finishing annealing.
The grain-oriented silicon steel sheets obtained in this manner usually have their surfaces coated with an insulating film composed mainly of forsterite (Mg2SiO4) (which is simply referred to as xe2x80x9cforsterite coatingxe2x80x9d hereinafter). This forsterite coating gives the steel sheets not only surface electrical insulation but also tensile stress resulting from low thermal expansion. Therefore, it improves iron loss as well as magnetostriction.
After final finishing annealing, grain-oriented silicon steel sheets are usually given a vitreous insulating coating (simply referred to as glass coating hereinafter) on the forsterite coating. This glass coating is very thin and transparent. Therefore, it is forsterite coating rather than glass coating that eventually determines the external appearance of the product. In other words, the appearance of forsterite coating greatly affects the product value. For example, any product would be regarded as inadequate if it had forsterite coating formed such that the base metal is partly exposed. Thus, the properties of forsterite coating seriously affect the product yields. That is, forsterite coating is required to have an uniform appearance without flaws, and with good adhesion to prevent peeling at the time of shearing, punching, and bending. Moreover, forsterite coating is required to have a smooth surface because the steel sheets laminated to form the iron core need to have a high space factor.
There have been disclosed various technologies to improve the magnetic properties of grain-oriented silicon steel sheets. One of them involves the use of an auxiliary inhibitor that makes up for the function of the main inhibitor such as MnS, MnSe, AlN, and BN. Among the known elements which function as auxiliary inhibitors are Sb, Cu, Sn, Ge, Ni, P, Nb, V, Mo, Cr, Bi, As, and Pb. Of these elements, Bi is known to give a much higher magnetic flux density than before (For example, Japanese Patent Publication Nos. 32412/1979 and 38652/1981, Japanese Patent Re-publication No. 814445/1990, Japanese Patent Laid-open Nos. 88173/1994 and 253816/1996). However, adding Bi to steel presents difficulties in producing good forsterite coating at the time of finishing annealing. Products with poor coating are usually rejected.
Forsterite coating is formed at the time of final finishing annealing. The formation of forsterite coating affects the decomposition of inhibitors (such as MnS, MsSe, and AlN) in steel. In other words, it also affects the secondary recrystallization which is an essential step to obtain good magnetic properties. In addition, forsterite coating absorbs the components of inhibitor which become unnecessary after the completion of secondary recrystallization, thereby purifying steel. This purification also contributes to improvement in the magnetic properties of steel sheets.
Consequently, forming a uniform forsterite coating by controlled steps is very important to obtain grain-oriented steel sheets with good magnetic properties.
Forsterite coating is usually formed by the following steps. First, a grain-oriented silicon steel sheet which has been cold-rolled to a desired final thickness is annealed in wet hydrogen atmosphere at 700-900xc2x0 C. This annealing is called decarburization annealing. It has the following functions.
(1) To subject the texture (after cold rolling) to the primary recrystallization so that the secondary recrystallization takes place adequately in the final finishing annealing.
(2) To reduce the content of C in cold-rolled steel sheets from about 0.01-0.10 wt % to about 0.003 wt % or less so as to protect the magnetic properties of the product from aging deterioration.
(3) To cause subscale (containing SiO2) to form in the surface layers of steel sheets by oxidation of Si that is present in steel.
After decarburization annealing, the steel sheet is coated with an annealing separator (composed mainly of MgO) and then coiled. The coil undergoes final finishing annealing (which serves also for secondary recrystallization and purification) in a reducing or non-oxidizing atmosphere at about 1200xc2x0 C. (maximum). Forsterite coating is formed on the surface of steel sheet according to the solid-phase reaction shown by the following formula.
2MgO+SiO2xe2x86x92Mg2SiO4
Forsterite coating is a ceramic coating densely composed of fine crystalline particles about 1 xcexcm in size. As the formula shows, one raw material of forsterite coating is subscale containing SiO2 which has formed in the outer layer of the steel sheet at the time of decarburization annealing. Therefore, the kind, amount, and distribution of subscale are deeply associated with the nucleation and grain growth of forsterite coating. They also greatly affect the strength of grain boundary and grain of coating crystals and further affect the quality of coating after final finishing annealing.
The annealing separator (composed mainly of MgO as another raw material) is applied to the steel sheet in the form of an aqueous slurry. Therefore, steel sheets retain physically adsorbed water even after drying, and MgO partly hydrates to form Mg(OH)2. As the result, steel sheets continue to give off water (although small in quantity) until the temperature reaches about 800xc2x0 C. during final finishing annealing. This water oxidizes the surface of the steel sheet during final finishing annealing. The oxidation by water also affects the formation of any forsterite coating and the behavior of inhibitors. Added oxidation by water is a factor tending to deteriorate magnetic properties. In addition, the ease with which oxidation by water takes place depends greatly on the physical properties of subscale formed by decarburization annealing.
Also, any additives other than MgO incorporated into the annealing separator, however small in quantity, greatly affect the film formation as a matter of course.
In the case of grain-oriented silicon steel sheets having a nitride inhibitor (such as AlN and BN), the physical properties of subscale greatly affect the behavior of denitrification during finishing annealing or the behavior of nitrification from the annealing atmosphere. Therefore, the physical properties of subscale greatly affect the magnetic properties of the sheet.
As mentioned above, controlling the physical properties of subscale formed in the outer layer of steel sheets during decarburization annealing, controlling the properties of magnesia in the annealing separator, and controlling the kind of additive in the annealing separator are three factors indispensable in forming forsterite coatings of uniform good quality at a prescribed annealing temperature which is determined by the condition of secondary recrystallization in finishing annealing. They are very important in the production of grain-oriented steel sheets.
Incidentally, if the steel does not contain Bi, forsterite coating of good quality may be formed by any of the disclosed techniques given below.
Japanese Patent Laid-open No. 185725/1984, controlling the oxygen content in steel sheets after decarburization annealing.
Japanese Patent Publication No. 1575/1982, keeping the degree of oxidation in the atmosphere at 0.15 and above in the front region of decarburization annealing and at 0.75 and below in the rear region that follows.
Japanese Patent Laid-open No. 240215/1990 and Japanese Patent Publication No. 14686/1979, performing heat-treatment at 850-1050xc2x0 C. in a non-oxidizing atmosphere after decarburization annealing.
Japanese Patent Publication No. 57167/1991, cooling after decarburization annealing in such a way that the degree of oxidation is lower than 0.008 in the temperature region below 750xc2x0 C.
Japanese Patent Laid-open No. 336616/1994, performing heat treatment in such a way that the ratio of the partial pressure of water vapor to the partial pressure of hydrogen is lower than 0.70 in soaking step and the ratio of the partial pressure of water vapor to the partial pressure of hydrogen in the heating step is lower than that in the soaking step.
Japanese Patent Laid-open No. 278668/1995, prescribing the rate of heating and the atmosphere of annealing.
Forsterite coating looks poor if the base metal is exposed sporadically. This defect can be avoided by the method disclosed in Japanese Patent Laid-open No. 226115/1984, which consists of causing the raw material to contain 0.003-0.1 wt % of Mo and performing decarburization annealing at 820-860xc2x0 C. such that the degree of oxidation in the atmosphere is 0.30-0.50 expressed as P(H2O)/P(H2), and the subscale formed on the surface of steel sheet is composed of silica (SiO2) and fayalite (Fe2SiO4), with the ratio of Fe2SiO4/SiO2 being in the range of 0.05-0.45.
Apart from the above-mentioned techniques relating to decarburization annealing, there have been proposed a number of techniques for improving the characteristic properties of the coating film. These techniques involve the addition of a Ti compound (such as TiO2), as an additive other than magnesia, to the annealing separator. For example, Japanese Patent Publication No. 12451/1976 discloses a method of improving the uniformity and adhesion of forsterite coating by incorporating 100 pbw of Mg compound with 2-40 pbw of Ti compound. Japanese Patent Publication No. 15466/1981 discloses a method of eliminating black spots from the Ti compound by finely grinding TiO2 for the annealing separator. Japanese Patent Publication No. 32716/1982 discloses a method of adding an Sr compound in an amount of 0.1-10 pbw (as Sr) so as to form forsterite insulating film with good adhesion and good uniformity.
Also, there have been disclosed several methods for improving the magnetic properties by adding a compound to the separator. Japanese Patent Publication No. 14567/1979 discloses the addition of Cu, Sn, Ni, or Co, or a compound thereof in an amount of 0.01-15 pbw (as metallic element). Japanese Patent Laid-open No. 243282/1985 discloses the addition of TiO2 or TiO (0.5-10 pbw) and SrS, SnS, or CuS (0.1-5.0 pbw), together with optional antimony nitrate (0.05-2.0 pbw).
Moreover, Japanese Patent Laid-open No. 291313/1997 discloses a method of improving both the magnetic properties and the film characteristics of the silicon steel product sheet. This method is based on the result of investigation on the relation between the subscale (which occurs at the time of decarburization annealing) and the annealing separator. The object is achieved by adjusting the partial pressure of hydrogen (P(H2)) and the partial pressure of water vapor (P(H2O)) in decarburization annealing such that the ratio of P(H2O)/P(H2) in the soaking step is lower than 0.70 and the ratio of P(H2O)/P(H2) in the heating step is lower than that in the soaking step, and also by incorporating 100 pbw of MgO in the annealing separator with 0.5-15 pbw of TiO2, 0.1-10 pbw of SnO2, and 0.1-10 pbw of Sr compound (as Sr).
There have been proposed other techniques developed, with attention paid to the amount of subscale in steel sheets which have undergone decarburization annealing. For example, Japanese Patent Laid-open Nos. 329829/1992 and 329830/1992 disclose a method of adding Cr and Sb simultaneously or adding Cr, Sn, and Sb simultaneously, thereby minimizing the fluctuation of the amount of oxidized layer and forming the coating film stably in finishing annealing. Japanese Patent Laid-open No. 46297/1989 discloses a method of making fayalite (Fe2SiO4) and silica (SiO2) thick enough for the formation of forsterite coating by adding Cr and establishing adequate conditions for decarburization annealing so as to promote diffusion of oxygen in the thickness direction.
Unfortunately, incorporating steel with Bi suffers difficulties in obtaining a good forsterite coating at the time of finishing annealing (which results in unacceptable products with poor coating film). In connection with this, Japanese Patent Laid-open No. 202924/1997 mentions that xe2x80x9cit is assumed that Bi vapor concentrated between steel sheets adversely affects the formation of primary coating, thereby making it difficult to form good primary coating film.xe2x80x9d Incidentally, this Japanese Patent discloses a method of increasing the magnetic flux density by the addition of Bi and also providing a material with low iron loss. (This method is based on the above-mentioned assumption.)
Even in the case of Bi-containing steel, good forsterite coating can be obtained by any of the methods disclosed as follows.
Japanese Patent Laid-open No. 232019/1996, adjusting the amount of oxygen in oxide film after decarburization annealing to 600-900 ppm and applying an annealing separator incorporated with 0.01-0.10 pbw of chlorine compound (as Cl) and/or 0.05-2.0 pbw of one kind or more than one kind of Bb, B, Sr, and Ba compounds, for 100 pbw of MgO.
Japanese Patent Laid-open No. 258319/1996, adjusting the amount of annealing separator (composed mainly of MgO) to 5 g/m2 or above on one side of steel sheet.
Japanese Patent Laid-open No. 111346/1997, adjusting the flow rate of atmosphere gas in finishing annealing such that the ratio of flow rate to the total surface area of steel strip is equal to or larger than 0.002 (Nm3/hm2).
Japanese Patent Laid-open No. 25516/1998, adjusting the Ig-loss value of magnesia in the annealing separator to 0.4-1.5 wt %.
Japanese Patent Laid-open No. 152725/1998, adjusting the amount of oxygen on the surface of steel sheet after decarburization annealing to 550-850 ppm.
Incidentally, the Ig-loss value is hydrate amount calculated by the weight difference between before and after baking process of making magnesia.
The above-mentioned techniques, however, do not basically change the reaction to form forsterite in the presence of Bi (or do not promote the forsterite reaction 2MgO+SiO2xe2x86x92Mg2SiO4). In other words, they do not improve forsterite coating satisfactorily, or they cannot stably form defect-free, uniform forsterite coating of good quality and good adhesion over the entire width and length of a coil product.
It is an object of the present invention to provide grain-oriented steel sheets superior in magnetic properties, having defect-free, uniform forsterite coating with good adhesion over the entire width and length of a coil even though the steel contains Bi in an amount of about 0.005-0.2 wt %.
The sheet has superior coating properties and magnetic properties. The process includes a series of steps of hot-rolling a silicon steel slab containing about C: 0.030-0.12 wt %, Si: 2.0-4.5 wt %, acid-soluble Al: 0.01-0.05 wt %, N: 0.003-0.012 wt %, Mn: 0.02-0.5 wt %, and Bi: 0.005-0.20 wt %, cold-rolling the hot-rolled sheet once or twice or more with intermediate annealing interposed, performing decarburization annealing to the final cold rolled sheet, applying an annealing separator to the surface of the decarburized steel sheet, and performing final finishing annealing consisting of secondary recrystallization annealing and purifying annealing to the separator-applied sheet, characterized in that the steel slab contains about 0.1-1.0 wt % of Cr so that a Cr spinel oxide is formed in the subscale oxide film in the surface layer of the resulting steel sheet when subjected to decarburization annealing.
In the above-mentioned process, the decarburization annealing may be accomplished in such a way that the decarburizing soaking temperature is about 800-900xc2x0 C. and the annealing temperature is increased at an average rate of about 10-50xc2x0 C./s from the starting temperature to 700xc2x0 C. and then the temperature is raised at an average rate of 1-9xc2x0 C./s from (soaking temperature xe2x88x9250xc2x0 C.) to the soaking temperature.
In the above-mentioned process, the subscale Cr spinel oxide in the oxide film may be composed mainly of FeCr2O4 or (Fe,Mn)Cr2O4 or mixtures thereof.
In the above-mentioned process, the decarburization annealing may be accomplished in such a way that the amount of oxygen in the surface layer of steel sheet is about 0.35-0.95 g/m2 (on one side) and the annealed steel sheet has a surface thin film which is characterized in that the ratio of I1/I0 is about 0.2-1.5, where I1 is the peak intensity of X-ray diffraction due to (202) plane of FeCr2O4 or (Fe,Mn)Cr2O4 and I0 is the peak intensity of X-ray diffraction due to (130) plane of fayalite oxide.
In the above-mentioned process, the decarburization annealing may be accomplished in such a way that the degree of oxidation in the atmosphere at the time of soaking is about 0.30-0.50 expressed as P(H2O)/P(H2), and the degree of oxidation in the atmosphere exceeds by about 0.05-0.20 the atmosphere in the heating zone.
In the above-mentioned process, the annealing separator may contain about 0.5-15 pbw (in total) of one kind or more than one kind selected from SnO2, Fe2O3, Fe3O4, MoO3, and WO3 and 1.0-15 pbw of TiO2 in 100 pbw of magnesia.
Another feature of the present invention resides in the creation of a grain-oriented silicon steel sheet containing Cr and Bi as steel constituents and having a forsterite coating on the sheet surface, characterized in that the base iron and forsterite coating combined together contain about Cxe2x89xa630 wtppm, Si: 2.0-4.5 wt %, Al: 0.005-0.03 wt %, N: 0.0015-0.006 wt %, Mn: 0.02-0.5 wt %, Cr: 0.1-1.0 wt %, and Bi: 0.001-0.15 wt %.
A steel containing both Bi and Cr is found in Example 4 of Japanese Patent Laid-open No. 87316/1991. However, this Japanese patent merely discloses a steel containing only 0.009 wt % of Cr and mentions nothing about the properties of any coating. A steel containing 0.12 wt % of Cr and 0.083wt % or 0.0353 wt % of Bi is found in Example 3 of Japanese Patent Laid-open No. 269571/1996. The techniques in this Japanese patent is not intended to form a forsterite coating in view of the fact that the annealing separator, composed mainly of Al2O3, is applied afterward. Moreover, Japanese Patent Laid-open No. 269572/1996 discloses an experiment with a steel incorporated with 0.12 wt % of Cr and 0.007 wt % of Bi. The techniques in this Japanese patent relate to annealing for secondary recrystallization in the presence of a temperature gradient; the reference mentions nothing about the properties of coating film. In addition, Japanese Patent Laid-open No. 279247/1997 discloses an experiment with a steel incorporated with 0.12 wt % of Cr and 0.007 wt % of Bi. It gives only one example in which a steel incorporated with Cr is used and it mentions nothing about the effect of Cr on the properties of any coating film. In fact, it relates to a technology for the electrostatic spraying of an annealing separator that follows the application (followed by drying) of an aqueous slurry composed mainly of MgO. These disclosed techniques neither define the object (if any) of adding Cr nor even investigate any relationship between the properties of the coating and the addition of the Cr.